1. Field of the Invention
This invention relates to an image pickup apparatus for taking a picture of an image of an object such as a blackboard.
2. Description of the Related Art
When an explanation is given by use of the blackboard in a conference, it is sometimes required to easily record the contents written on the blackboard.
For this purpose, conventionally, a blackboard having a copying function, i.e., a so-called electronic blackboard is proposed. The technology of the electronic blackboard is disclosed in U.S. Pat. No. 4,687,318 and U.S. Pat. No. 4,733,271, for example.
However, since the electronic blackboard itself has the copying function, the size of the blackboard becomes large, making it difficult to be moved from one conference room to another.
Therefore, there is developed an image pickup apparatus which optically reduces the size of the image of an object such as a blackboard and takes the image of the object. More specifically, the image pickup apparatus derives an image signal by scanning the reduced image of the blackboard with a one-dimensional image pickup device, and prints the image signal on recording paper. Since the image pickup apparatus is a device different from the blackboard, the image pickup apparatus can be easily moved from one conference room to another. The technology of the image pickup apparatus is disclosed in U.S. Pat. No. 4,074,324 or U.S. Pat. No. 5,067,028, for example.
The exposure control for the above image pickup apparatus is effected by detecting the light exposure of an object image formed by a lens by use of a light receiving element different from the image pickup device and controlling the diaphragm mechanism according to the detected light exposure. In some cases where a two-dimensional image pickup device is used as the image pickup device, the diaphragm mechanism is controlled according to the light amount detected by the two-dimensional image pickup device.
However, the light amount is irregular on the imaging plane of the lens, and the effect of this irregularity cannot be eliminated. As a result, adequate light exposure control cannot be made if the light exposure in an area larger than a preset area on the imaging plane is not detected in a case where the image pickup device is used.
Of course, in the image pickup apparatus using the two-dimensional image pickup device for image pickup, the light amount on the entire imaging plane can be detected by use of the two-dimensional image pickup device so that no problem will occur even if the light exposure control is performed using the image pickup device. However, in the image pickup apparatus using a one-dimensional image pickup device, the light amount can be detected only in a one-dimensional fashion and the light amount of more than a preset area cannot be detected even if the one-dimensional image pickup device is used in the same manner as in the case of the two-dimensional image pickup device to effect the light exposure control.
Therefore, in the image pickup apparatus using the one-dimensional image pickup device, it is difficult to control the light exposure by using only the one-dimensional image pickup device. It then becomes necessary to additionally use a light exposure sensor for detecting the light exposure in order to adequately perform exposure control.